Transmission control in signaling systems



Nov. 9, 1937. o, R, GARFIELD l2,098,2@5

TRANSMISSION CONTROL IN SIGNALING SYSTEMS Filed July 31, 1936 A TTORNEV Patented Nov. 9, 1937 PATENT oFFlCE TRANSMISSION CONTROL IN SIGNALING SYSTEMS Owen 2R. Garfield, Floral Park, N. Y., assigner to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application July 31, 1936, Serial No. 93,584

20 Claims.

The invention relates to signal wave transmission systems and particularly to wave-operated switching circuits for controlling signal transmission in such systems.

I 5 The invention has particular reference to socalled Codan (carrier-operated detector antinoise) circuits employed in connection with twoway signaling systems, for example, two-way vradio telephone systems, for disabling the receiving circuit at each terminal station during periods .of no signal transmission and removing the disability when signal transmission is taking place. 'Ihis prevents the reception of interference (static) which, during periods or" no signal` (no carrier) when automatic gain control receivers are at maximum sensitivity, may be of sufficient amplitude to annoy the receiving subscriber, and to falsely operate the voice-operated switching circuits for controlling thetransmission of carrier by the transmitter at the terminal. The lusual Codan circuit is arranged to be controlled by the unmodulated carrier, but may be controlled by control energy of distinctive frequency other than the carrier, sent out from the transmitting station under controlof the signal waves to be transmitted.

Static may be considered to consist of a more or less steady noise on which is superimposed irregular noise crashes of varying amplitude and shape. Over long periods the long-time static power may vary over a wide range, 50 decibels or more, and the peak factor may vary over a smaller range, of decibels or so.

It is an object of the present invention to improve the operation of such Codan circuits when subjected to interfering waves or static of a Wide range of levels and of widely different characteristics.

A more specic object of the invention is to permit unattended Codan operation, that is, to

automatically control the sensitivity of a Codan in accordance with the static level so as to reduce the probability of its false operation by static While maintaining it at maximum sensitivity as regards operation by carrier.

These objects are attained in accordance with the invention by employing for the carrier or control energy-operated circuit for operating a switching relay to remove a normal disability in the output of the receiving circuit, during periods of signal transmission, a circuit which will effectively suppress from that relay both slow and fast variations in static when the carrier is not present, and which, when the carrier is present, will act as a static desensitized detector, that is,

(Cl. Z-6) a detector which is very sensitive to the carrier whenthe static level is low and the sensitivity of which decreases approximately linearly as the kstatic level rises. For a given level of static, it is only necessary that this detector operate on carrier strong enough to give a commercial circuit, that is, a circuit in which the signal overrides the received static by a sucient amount to be understood. In one embodiment this circuit comprises two control circuits in tandem. The rst consists of an amplier, the gain of which is adjusted by a relatively slow backwardacting control functioning on slowly varying static to make the average static output of the amplifier constant. Thesecond control circuit comprises a fast forward-acting desensitizing detector with associated filters, for suppressing the fast variations in static from the .control relay in its output. The two control circuits are arranged to transmit quickly to the controlr relay control waves of sulicient amplitude to cause its operation.

The objects and advantages of the invention will be better understood from the following detailed description thereof when read in connection with the accompanying drawing,

Fig. 1 of which shows diagrammatically a twoway radio telephone system embodying the invention; and l Figs. 2 to 4 of which show schematically circuits which may be used for the various elements of the circuit o1"` the invention in the system of Fig. 1.

The diagram of Fig, 1 is not an actual circuit diagram but rather a single line layout, each line indicating a two-wire transmission path. A break in a path, represented by separatedarrowheads, indicates that the path is normally disabled at that point, and an arrow pointing toward such a break from a control device (amplifier-detector) indicates that the path will be made operative by operation of the control device. A make in a path, represented by contacting arrowheads, indicates that the path is operative at that point, and an arrow pointing. toward the contacting arrowheads from a control device (ampliler-detector) indicates that the path will be disabled at that point by operation of the control device. In the diagram, the triangular boxes represent amplifiers, and an arrow directed at such a box from a control device (detector) indicates that the gain of the amplier will be adjusted under control of that control device. Y

Fig. V1 represents a two-way radio telephone system for transmitting telephone signals in oppcsite directions between a west two-way telephone line LW and an east telephone line LE.

The west terminal of the radio telephone system comprises the transmitting circuit TW including a transmitting antenna TAN, and a receiving circuit RKK/including a'receiving-` antenna RAW, the input ,of the circuit TW andthe output of the circuit RW being coupled in conjugate relation with each other and in energy transmitting relation with the two-way telephone line LW by the hybrid coil connectionl-Ii and balancing network N1 in well-known manner.

Similarly, the east terminal of Vthe radio telephone system comprises the transmitting circuit TE including the transmittingantenna TAE, and the receiving circuit RE including the receiving antenna RAE, the input of the circuit TE and the output of the circuit RE being coupled in conjugate relation with each other and in energy transmitting relation with Vthe two-way telephone line'LE by the hybrid coil H2 and associated balancing network N2.'

The transmitting circuit TW' at the west terminal comprises the usual transmission apparatus including the radio transmitter I and associated carrier oscillator 2. In the absence of speech wave transmission therein, the'circuit TW is 'arranged in any suitable manner, as indicated by the normal open-circuit 3 between transmitter I and transmitting antenna TAW, to prevent the radiation of carrier oscillations to the east terminal station. When speech wave signals are received in the transmitting circuit TW from the line' LW, a portion thereof is impressed on the input of the wave-operated control device (ampliler-detector) 4 bridged across the `circuit TW in front of the radio transmitter I, Yand causes the operation of'that device to remove the disability in the circuit TW at the point 3, thereby allowing signal-modulated carrier waves, as well asthe unmodulated carrier, to be radiated by 'the transmitting antenna TAW to the east terminal of the system.

The receiving circuit RE in the east terminal of the system comprises the usual transmission apparatus including the antenna tuning means 5, which may include'some fixed gain, the radio amplier 6, the detector 1, the automatic gain control `8 for the radio amplifier 6, controlled-by ,the receivedcarrier from vthe output of detector 1, for regulating the sensitivity of thev receiving circuit RE to compensate for variationsin received signal volume, and the audio amplier 9. The automatic gain control 8 may be of any of the vtypes well known in the art.

Associated with the receiving circuit RE of 'the east terminal is the Codan circuit I0 of the invention. As indicated, it comprises two control circuits in "tandem, the rst consisting of the Variable gain Aamplier II having its input connected across the receiving circuit RE in front of the radio amplifier 6, and a backward-acting circuit I2 comprising the lter'Fi and rectier I3 for controlling the gain of amplier II; and the second consisting of a forward-acting circuit comprising the two parallellters F2 and F3 fed from the output of the amplifier II, and a detector I5 in the output thereof which, when operated, renders the output of the receiving'circuit RE, normally disabled at the Vpoint I5 as indicated by the separated arrowheads, operative at that point. Y

The filter F1 in the backward-acting control circuitv I2 is designed to exclude the carrier and carrier frequency and side-bands.

The transmitting circuit TE at the east terminal of the radio telephone system of Fig. 1

- contains transmission and control apparatus similar to that used in the transmitting circuit TW at the west terminal of the system, as indicated x by the use of similar characters for identifying the corresponding pieces of apparatus, except that in the circuit TE the characters are followed by a prime mark. Also, the receiving circuit RW at the west terminal of the radio telephone system contains transmission and control apparatus similar to that used in the kreceiving circuit RE at the eastterminal, including a Codan circuit, as indicated by the use of the same characters foridentifying the corresponding pieces of apparatus in the receiving circuit at each terminal, exceptthat these characters in the case of the apparatus in the receiving circuit RW are followed by a prime mark. However, the carrier frequency generated by the carrier oscillator 2 in the transmitting circuit TE at the -east terminal may be different from the carrier frequency generated by the carrier oscillator 2 at l the west terminal, in which case the lters in the Codan circuit I0 at the west terminal correspending to the lters F1, F2 and F3 in the Codan circuit I0 at the east terminal, may have frequency pass ranges determined by the particular value of frequency selected for the carrier oscillator 2 .at the east terminal.

For the purposeof givingexclusive control of the switching apparatus at both terminals to the particular subscriber associated with the telephone line LW or LE, who first starts to talk, additional switching apparatus is provided at both terminals as follows. Operation of the control circuit (amplifier-detector) 4 in response to telephone signals in the circuit TW, in addition to rendering theflatter circuit operative at the point 3, also disables the normally operative trolling the transmission of carrier from the eastA terminal, to be disabled in its input, and operation of the Codan circuit III at the west terminal,

when its output is operative, causes the control circuit (amplifier-detector) 4 controlling the transmission of carrier from that terminal, to be disabled in its input, as' indicated.

The system of Fig. l operates as follows: When no speech signals are being received by the radio telephone system from the telephone line LW, associated with the west radio terminal, or by the telephone line LE, associated with the east radio terminal, the control circuits at the two radio terminals are in the condition indicated in the ligure, with both transmitting circuits TW and TE disabled in theiroutputs at the points 3 and 3", respectively, so that no carrier waves 'can be radiated to the other terminal, and with the receiving circuits RE and RW blocked in their outputs at the points I6 and I6', re-

the transmitting circuit to the switching apparatus for controlling the transmission of carrier from that terminal. The action of the automatic gain control in each receiving circuit is to adjust the latter to maximum sensitivity7 during non-signaling intervals, so it is apparent that the noise currents, if they are allowed to go through, may be of sufficient amplitude to greatly annoy the subscriber and to cause false operation of the carrier control apparatus at the terminal.

Now, let it be supposed that speech waves are being received in the transmitting circuit TW of the west radio terminal from a subscriber associated with the telephone line LW. The main portion of these waves will be impressed on the input of the radio transmitter 2 and will modulate the carrier frequency from the associated carrier oscillator 2 therein in well-known manner. A portion of the speech waves in the input of the radio transmitter I will be diverted therefrom and will be impressed upon the control device causing its operation to remove the disability at the point 3 in the output of the transmitted circuit TW, so as to allow the speech signal modulated carrier, as well as the unmodulated carrier waves, to be radiated by the transmitting antenna TAW to the eastterminal of the radio telephonev system.

Y The operation of the control circuit 4 also disables the output of the Codan circuit I associated with the receiving circuit RW atthe west terminal thereby preventing the disability at the point I 6' therein from subsequently being removed by carrier waves received from the east terminal.

At the east terminal of the radio telephone system, the carrier waves and signal-modulated carrier will be picked up by the receiving antenna RAE and selected by the antenna tuning device 5. The selected waves will divide between the input of the radio amplifying device 6 and the input of the amplifier II in the Codan circuit I.

IThe speech signal-modulated carrier waves in the output of the amplifier 6 will combine in the detector or demodulator 'l with the unmodulated carrier to produce the speech waves in well-known manner. Also, a portion of the rectied waves will be fed back from the detector 'l through the connection 8 to adjust the gain of radio amplifier 6 in accordance with the amplitude level of the received waves in wellknown manner. The detected speech signal waves in the output of the detector 'I will then be amplified by audio amplifier 9.

The portion of the waves fed into the Codan circuit lil from the output of the antenna tuning device 5 in receiving circuit RE will include the Y waves in the output of amplifier I I is fed into the backward-acting control circuit I2. The lter F1 in that control circuit suppresses the carrier and side-bandaand the remaining frequencies includingthe amplified static, are fed into the rectifier I3, the rectified output of the latter being used to control the-gain of the amplier II in such manner as to make its gain decrease for increases in input level and to increase for decreases in the input level.

The combination comprising the amplifier II and the backward-acting control I 2 including filter F1 and rectier I3, for controlling its gain, would be inherently slow in operation, since a particular peak of static must have passed through the amplier before that peak begins to control the amplifier gain. By proper selection of the time constants of the constituent elements, the operation of the combination is slowed up still more and made to function only on the long time static power, or more simply, on the long time rectified average of the static applied thereto. This means that the combination will be-eifective to suppress slow changes of static level, but the static peaks of varying amplitudes, such as sudden crashes, will go through the device practically unchanged in relative size. Such a backward-acting device can easily be made to function over a static amplitude range of 50 decibels, and to hold the long-time rectified average of the static to as close limits as desired.

The control circuit i4 connected in tandem with the backward-acting device, constituting a forward-acting desensitized detector operating on peaks and having substantially no speed limitation, is used to supplement the former device over the range of fast varying static. Such va device would not be satisfactory as a Codan, if used alone, since it would be extremely dicult to build a control that would function over the complete static range ofY approximately 70 decibels, but a control which will take care of the smaller range, kabout 20 decibels, of the fast variations in'static, may be easily made.

The amplified waves in the output of amplifier li, from which the slowly varying static variations have been removed are transmitted -into the control circuit fl and are impressed on the parallel ilters F2 and Fs therein. As previously stated, lthe two filters F2 and Fa are similar'in characteristic, except that one, F2, passes the carrier frequency whereas the other, F3, does not.

YThe outputs of the two lters F2 and F3 are fed into the input of detector I5, which would preferably comprise two rectiiers the outputs of which are connected in phase opposition, which detector, when operated, removes the disability from the receiving circuit RE at the point I6. This forward-acting deviceworks on the principle that steep wave fronts applied to two similar lters separated slightly as to their frequency pass ranges, will produce disturbances at the outputs of the two `filters having very similar envelopes. Thus,'the outputs of the two rectiers in detector I5 tend to neutralize each other since they are in phase opposition, as described more fully later in connection with Fig. 2.

The carrier frequency, however, passed by the lter F2 is applied to the input of detector I5 in such manner as to cause its operation to remove the disability at the point I6 in the output of the receiving circuit RE, thereby allowing the amplied speech waves'in the output of the audio amplifier 9 to be transmitted out over the output side-band alone.`f rA' portion of the amplified oi the: circuit RE through the hybrid coil H2 to the line. LE. over which it will be transmitted to the telephone subscriber. The detector I5 is designed to have a small time constant so that it will` operate quickly in response to the applied carrier toi make the receiving circuit RE operative and thus to prevent any speech energy from being lost in the circuit RE.

It should be noted that the amount of carrier.

this' sensitivity is a maximum for the amount of static` present.

Operation of the detector I5 in response tothe applied carrier, in addition to removing the disability at the point I6 in the output of the receiving circuit RE, also causes the control circuit (amplifier-detector) 4 associated with the transmitting circuit TE at the terminal, to be disabled in its input, thus preventing it from being falsely operated thereafter by any energy Which may be transmittedthrough the hybrid coil H2 from the output of RE to the input of TE, to make the latter circuit operative to radiate that energy as well as` the carrier from the source 2.

When speech waves applied from the line LW to the west radio terminal cease, the control device 4 associated with the transmitting circuit TW will release, redisabling the output of the circuit TW at the point 3, and thereby preventing carrier oscillations from the oscillator 2 from being radiated thereafter to the east terminal of thc system, and rendering the output of the Codan IB at the west terminal operative. The detector I5 in the Codan circuit I0 at the east terminal will also release on cessation of the supply of carrier Waves thereto, causing the receiving circuit RE at the east terminal to be redisabled at the point IE in its output, and the disability to be removed from the input of control circuit 4 at the east terminal. The control devices at the two terminals may be given suitable hang-overs in operation by methods well-known in the art, sothat clipping of the speech waves by premature release of the control devices will be minimized. Y

A schematic circuit of one type of detector' which may be used for the detector I5 in the system of the invention in Fig. 1, is shown in Fig. 2. As indicated, this detector includes a two-element rectifier tube I1 in series with a resistance I8 and a condenser I9 in parallel, coupled across the output of the filter F2, and a two-element rectifier 20 in series with the resistance 2I and condenser 22 in parallel, coupled across the output of the filter F3. The two rsistances I8 and 2| are connected in series across the control gridcathode circuit of the screen grid direct current amplifier tube 23, and the winding of a mechanical relay 24 is connected in the platecathode circuit of the tube 23.

With these connections, the static outputs of the lters F2 and F3, having substantially the same envelopes, which are'the static peaks not suppressed by the preceding backward-acting control circuit, produce opposing substantially equal opposing voltage drops in the series resistances I8 and 2| in the grid circuit of the tubeV 213,` and,` therefore,` are substantially neutralized so as, to prevent operation ofl the tube 23 by static. 'I'heoperating sensitivity. of the tube 23 will be decreased in proportion to the amount of voltage4 drop produced by the static. The carrier frequency, however, being passed by the filter F2 only, produces a voltage drop'in resistance I9 which biases the control grid of the amplifier tube'23 inthe operating direction, and if Vthe carrier energy is sufficient to'overcome the reduction in sensitivity occasioned by the balancing out of the particularstaticpeaks present at the time the carrier arrives will cause the operation of tube 23 to energize relay 24 in its output thus causing the operation of that relay. The operation of relay 24 to open its normally closed armature and contacts removes the normal disabling short-circuit across the output of circuit RE at the point I6'.

By giving suitable values to the elements of thev condenser-resistance combination, I9, I8 and 22; 2l, the detector circuit may be made to have a small time constant and thus produce the required quick operation on applied carrier. When static and carrier are both applied to the detector tubes I1 and 20, the effective sensitivity of the detector circuit depends on the amplitude of the static peaks present, the amount of carrier necessary to operate the detector circuit depending approximately on the short-time peak factor of the static.

Fig. 3 shows schematically the circuit of one type of amplifier which may be used for the variable gain amplifier IIin the Codan circuit I0 of Fig. 1, and Fig. 4 shows schematically one type of rectifier circuit which may be used for rectier I3 in the, system of Fig. 1 to control amplifier II.

The amplifier I I of Fig. 3 comprises three vacuum tube stages 25, 26 and 21, which are preferably of the pentode type as shown. A coupling network 28 in the input of the first stage 25, which in the system of Fig. l wouldV couple the amplier to the circuit RE, and the coupling networks 29 and 30 between vacuum tube stages 25 and 26, and stages 26 and 21, respectively, are designed to produce a band-pass filter effect to make the amplifier transmit eiiciently to the lters Fi, F2Y and Fs coupled to the last stage 21, a band 01` frequencies wider than that necessary for carrier and sidebands. The varying potential from the output of rectier I3 (Fig. 4) transmitted by way of conductor 3l through the resistances 32, 33` respectively in the control gridcathode circuits of tubes 25 and 26, varies the bias on .the control grids of these tubes to control the gain of the amplier.

The control rectier circuit of Fig. 4 comprises a two-element vacuum tube rectifier 3G having its input coupled to the output o1 the filter Fi, and two three-electrode vacuum tube stages 31 and 38 of direct current amplification. The parallel-connected resistance 39 and condenser 40 in the output of rectifier 36 operate as a iilter for the rectier output voltage to reduce alternating current ripple, the condenser also increasing the eiciency of rectification. The input electrodes of the first vacuum tube stage 31 of the direct current amplier are connected across the resistance 39 so that the ltered output voltage of the rectifier is impressed on the direct current ampliiicr.

The battery 4I through resistance 39 negatively biases the grid of tube 31. This battery is made greater than necessary to cut off tube 31 'trol of the operate and release times of the circuit.

The input electrodes of the second vacuum tube amplifierl stage 38 are connectedacross condenser 44. Plate current is supplied to the plate of tube 38 from battery 45 through the large series .resistance 48. A'condenser t1 connected between the plate and cathode of tube 38 is provided for giving further control of the operate and release times of the circuit. One terminalof the condenser 41 is connected through conductor 3| and the filtering resistances 32 and 33 in parallel respectively to the control gridcathode circuits of the amplifier tubes 25 and 26 vOf Fig. 3.:

Connection 3| includes the battery 48 for producinga desired normal negative bias on the control grids of amplier tubes 25 and 26.

The circuitof Fig. 4 operates to control the gain ofthe amplifier of Fig. 3 in the following manner: Y

' The Waves in the output of the lterFi, which will include all frequencies in the output of the amplifier of Fig. 3 except the carrier and sideband frequencies suppressed by that filter, will be rectied by rectier 36, the time constant of resistance 39 and condenser 40 governing how long-` an average rectied static is used for control. The rectified-voltage will be impressed on the ampliertube 31 in such manner as to op- Y pose ythe negative bias on the control grid of that tube.

.When the co-ntrol grid of tube 31 becomes suiiciently less negative, its plate impedance decreases causing the condenser 45 in its platecathode circuitfto become charged because of thecurrent flowing through the resistance 43. AAs condenser 44 charges, more and more negative bias is applied to the grid of the amplier tube The plate impedance of the tube 38 increases as the grid-voltage becomes more negative, and the vvoltage drop in the resistance @6 in its plate circuit, due to the output current of tube 38, decreases, causing a slowly increasing negative bias to be applied to the grids of the tubes 25 and 26 in the amplifier of Fig. 3, through connection 3| and the resistances 32 and 33 in the respective control grid-cathode circuits ofthe two tubes. The time constants of the circuit of Fig. l4, by proper selection of the values of the various condensers and resistances therein, are

ymade such that the gain of the -amplier of Fig.

3 will be a function of the average rectiiied static, so `that the long-time average of static at its output will beheld nearlyA constant. The gain will decrease relatively slowly for sudden increases in static and will increase faster when the static suddenly decreases. Thus the slow variations in static level Vwill be suppressed in carrier to the forward-acting. control circuit h3 of the Codan It! of Fig. 1. The latter operates in a manner which has been previously described in detailin `connection with Fig; 2, to suppress the fastvariations in static also, while allowing the carrier waves to operate the Codan relay 24 quickly, and thus to remove the disability at the point I6 in the receiving circuit of the vvradio terminal during periods of signal transmission.

Various modications in the circuits which A have been illustrated and described which 4are Within the spirit and scope of the invention will be apparent to persons skilled in the art. For example, the lter F1 in the backward-acting con'- 'trol circuit of Fig. 1 may be eliminated if the input of the rectifier I3 controlling thegainY of amplier l, is connected across the output of the iilter F3 in the forward-acting control circuit I4, instead of directly across the output of amplier inasmuch as the filters F1 and F3 both are designed to suppress the-carrier frequency.

What is claimed is: y

1. A receiving circuit for a carrier wave signaling system subject to static, comprising a. signal demodulating path normally disabled in its output, a wave-operated switching circuit responsive to the received carrier waves to removethe dis'- ability from said path While signals are being received, and means to minimize the eiect of static on the operation of said switching circuit vcom'- prising one static controlled means unresponsive to received carrier waves :for suppressing from said signaling circuit the slow variationsl in static and another means for substantially suppressing therefrom the fast variations in static while controlling the sensitivity ofsaid switching circuit in accordance with the applied static variations so as to maintain it at all times at maximum sensitivity to the carrier waves for the amount 'of static present.

2. The circuit of claim l, in which said one means comprises means selectively responsive to applied static to automatically change the gaing of said switching circuit in accordance with the slow variations in static.V

3. The circuit of claim 1, in which said one means comprises an ampliiier in the input of said switching circuit, the gain of which is automatically varied to maintain the average level of static in the output thereof, over a given period of time, substantially constant.

4. The circuit of claim 1, in which said one means comprises an amplifier in the input of said switching circuit, and means responsive to the long time rectied average level of the static in the output of said amplifier to adjust its gain.

5. The circuit of claim 1, in which said one means comprises a Wave amplier in the input off said switching circuit, and means for controlling its gain so that the gain increases relatively fast as the amplitude level of the waves in the input of the amplier is decreased and the gain decreases slowly when theamplitude level of the waves in the input increases.

6. The circuit of claim 1, in which said other means comprises a forward-acting detector in said switching circuit, the sensitivity of which is varied under control of the applied static peaks in accordance with their amplitudes.

'7. The circuit of claim 1, in which said other means comprises a wave operated device which when operated removes said disability from said path, two lters supplied with the received carrier waves and the received static after the slow variations have been removed therefrom, and two rectiers respectively supplied with the outputs 'of said two filters, the outputs of said rectiiiers said device, said filters being of like characteristics except that only one passes the carrier wave frequency.

8. In a signal ywave transmission system comprising a signal wave transmission medium subject to variable interefering noise waves of a lwide `range of amplitude levels, a signal transmitting circuit coupled lto the input of said medium anda lsignal receiving circuit coupled `to its output,

means normally disabling said receiving circuit in its output, means for sending out over said medium to said receiving circuit during signal transmission only, control waves of a frequency different from the signal frequencies, a wave-operated switching circuitresponsive to the received control waves to remove the disability from the output of said receiving circuit, means v'for preventing false operation of said switching circuit by the received noise waves over a wide lrange of levels while continuously adjusting its rsensitivity to maintain it at maximum sensitivity to said control waves yfor the level of znoise waves present at any time, and auxiliaryrneanszfor `.making said switching circuit quick-operating :in response to applied control waves.

9. In a signal wave transmission system comprising a signal wave transmission 'medium :subject to variable interfering noise waves of 'a wide range of levels, a Vsignal transmitting circuit coupled to the input of said medium and a signal receiving circuit' connected to the Voutput thereof, means normally disabling'said receiving circuit in its output, means responsive Vto the initiation in said transmitting circuit of signal Waves for transmission, to cause control iwaves of a frequency different from the signal frequencies, to be transmitted to said receiving circuit, a 'wavevoperated .circuit rconnected to `said receiving circuit so 'as to be supplied with v.the control and noise waves received thereby, Aswitching vmeans `responsive to operation "of said Wave-operated circuit to remove the disability 'from' the voutput ,of said receiving fcircuit, and meansfor substantially desensitizing Asaid wave-operated vcircuit as .regards operation by Athe received noise waves over a wide range of levels while allowing `itsquick and positive operation by 'the applied control waves.

10. "Ihe ksystem of claim 9, in which `said ldesensitizing :means comprisesmeans 'for separately removing effectively the slow and fast variations in level from the noise `Waves applied to `said wave-operated circuit.

11. The system of `claim 9, 'in whichsaid desensitizing means 'comprises v-means for reducing the noise `waves in vthe applied 'wavesto a :constant 'average level over a given long period o'f time, and means for'theneffectivelyzremoving from the -resulting waves the noise peaks before applying `thern to :said wave-operated circuit.

12. 'I'he system of claim 9,1in which said -desensitizing Vmeans includes an amplifier the .gain of which is "automatically controlled Iby the applied noise waves in rsuch 'manner `as to make the `average `noise output thereof `over a long period of time substantially constant, `meansY for sep- 'arating the noise lpeaks and the vcontrol Wave 'components in the foutput waves `of said ampli- Yreceiving circuit normally disabled Vnear its cutput, connected to said medium, means for sending out over said medium to saidreceiving circuit control waves of a frequency different Vfrom 'the transmitted signal frequencies, a 'wave-.operated switching circuit responsive to theicontrolwaves received in Ysaid receiving .circuit fto remove the -disability from theoutput thereof, and .means for fprevent-ing'false operation of said switching Lcircuit by received noise waves comprising two control circuits in tandem in the input thereof 'fone control circuits being Varranged to l'transmit the I received control waves quickly to said Aswitching circuit.

14. The system of claim 13, in which said-one control circuit comprising an amplifier and' means for controlling its gain so 'thatthe gain :increases relatively fast as the input level is fdecreased `and the 'gain decreases slowly whentheinput level increases. y

15. The system of claim 13, in which saidone control circuit comprisesan amplifier yand afeedback circuit coupling its output `'and input c'ircuits for controlling itsgain, including ailter :for suppressing the control Awave frequency and v`a rectifier. Y 16. The system of claim 13, in whichsaidfother control circuit comprises -a detector, `lcwo lters connected to the output of said one lcontrol means, and feeding in phase 'opposition intofthe-input Vcircuit of said detector, said filters being similar in characteristic except that only one passes tthe control wave frequency, said detector having a small time constant so as to 'operate quickly in response to the application of said control "wave thereto from the output of said one filter.

17. In a carrier telephone system comprisinga transmitting and a receiving 4station connected by a transmission medium subject 4to-staticofwidely varying level, means at the transmitting station for transmitting carrier energy over 'said medium to said receiving station during periods of signal transmission only, a receiving circuit at said receiving stationincluding carrier controlled automatic gain control means and signal demodulating means, said receiving .circuit being normally disabled in its output, a wave-operated switching circuit bridged across said receiving circuit in front of said automatic gain control means so as to besuppled with the received carrier energy and static therein, said switching circuit comprising an amplifier, a comparatively slow backward-acting circuit for controlling its gain in such manner that as the input level decreases the gain of the amplifier increases relatively fast and as the input .level increases 'the' gain of the amplifier decreases slowly, ,a detector fed from the output of said amplifier, means -to suppress substantially the fast variations instatic from the waves applied to said detector while Vallowing transmission of the carrier `wave com- -ponents thereto to `cause operation of'said -de tector, and means `responsive to Aoperationof said detector to cause the normal disability "to ibe removed from the output of said receiving circuit.

18. The system of claimV 13,1in lwhich said one Cil control circuit consists of an amplifier and a control circuit coupling the amplier output and input and functioning on the noise Waves of slowly Varying level, only, to adjust the gain of the amplifier to make the average static output of said amplifier over a long time period substantially constant.

19. The system of claim 13, in which said other control circuit comprises a vacuum tube detec- 4 tor, two Wave filters in parallel fed from the output of said one control circuit, and having their outputs fed in phase opposition into the input of said detector, said filters having substantially identical frequency pass ranges except that one only passes the frequency of said control Waves,

and means responsive to the control Wave frequency passed by said one filter to cause operation of said detector to remove the disability from the output of said receiving circuit.

20. The system of claim 13, in which said one control circuit comprises an amplier and a feedback circuit coupling its output and input circuits for controlling its gain, including a filter for suppressing the control Wave frequency and a rectifier, said feedback circuit Vhaving such time constants that thev gain of the amplifier increases relatively fast as the input level of the amplifier decreases, and decreases slowly as the input level increases.

OWEN R. GARFIELD 

